Directional Coherence Factor for Volumetric Ultrasound Imaging with Matrix Arrays.

Matrix arrays with small apertures limit spatial and contrast resolutions of volumetric ultrasound imaging. Coherence-based beamformers are prevalent for side lobe suppression and resolution improvement. While the spatial coherence of a matrix array is fundamentally a 2D function, conventional coherence factor (CF) methods neglect the directional variation of an M × N matrix array when calculating volumetric coherence. We hereby propose a projectionbased directional coherence factor (DCF) to exploit the 2D nature of volumetric coherence function. Instead of computing the coherent and incoherent summations across the entire 2D aperture, DCF projects aperture data onto azimuthal, elevational, diagonal, and antidiagonal directions and subsequently calculates the coherence factors for each direction separately. The orthogonal coherence pairs, i.e., azimuth and elevation, diagonal and anti-diagonal, are multiplied to obtain DCF_RC and DCF_Diag, respectively. The Jaccard similarity of the DCF_RC and DCF_Diag is used to derive the final DCF to weigh the reconstructed images. We evaluated the performance of DCF beamforming in point-target simulations, multi-purpose phantom experiments, and in vivo muscle imaging, and compared it to delay-and-sum (DAS) and CF beamformers. Our DCF achieved side lobe reduction throughout the entire volume compared to conventional CF. Moreover, diagonal weighting significantly improved, on average, the azimuthal resolution by 41.3% vs. DAS and 7.35% vs. CF as well as the elevational resolution by 40.4% vs. DAS and 38.7% vs. CF. Our proposed DCF offers a practical solution for resolution and contrast enhancement of volumetric imaging in 2D matrix array configurations.